Mined coal, being referred to as mineral coal, pit coal, hard coal and Steinkohle as is known, generally contains sulfur and mineral detritus. A portion of this sulfur is usually present in the form of pyrite (iron disulfide).
In power plants, boiling-firing application, gasification installations and the like, when mineral coal is used as a fuel, the coal is generally milled, classified to separate a fine component in the form of coal dust from a coarse component or gravel product containing larger pieces of coal to substantially all of the pyrite and a mineral detritus which may be present.
The coarse product is cycled to the mill and the coal dust is generally blown into a combustion chamber in ash stream.
Comminuted coal, produced in this manner, is especially effective for the firing of power plant boilers but also can be gasified, hydrogenated or utilized for other combustion purposes.
When the coal contains sulfur at least part of which is in the form of such pyrites, any combustion or gasification products will also contain sulfur unless the sulfur or pyrites have previously been removed.
Sulfur combustion products, namely sulfur oxides, can be generated in such combustion processes and pose a serious environmental danger because of their toxic, noxious and corrosive properties if vented to the atmosphere.
In many cases the combustion of high-sulfur coals is not permitted while in other cases such combustion is permitted but expensive energy-consuming methods must be used to remove the sulfur-containing components from the exhaust gas stream before it is vented to the atmosphere.
With some mineral coals, the problem is more pronounced than with others. For example in Ruhr Valley coal, some 40 to 60% by weight of the sulfur content of the mineral coal can be in the form of pyritic sulfur and pyrites themselves contain 50% and more sulfur by weight.
Generally the pyrite crystals are in a pure form embedded in or trapped in the coal and/or the coal and other mineral matter may be interlaced with or in the pyrite. A removal of pyrite from the mineral product can result in a substantial reduction in the sulfur content of the coal.
Various methods have been proposed to remove pyrite from coal. For example, it is known that subjecting the mined mineral product to high intensity magnetic fields will result in removal of pyrite, because of the magnetic properties of the latter, from the coal. These techniques are still under investigation and have not yet received widespread acceptance nor are they to be found in use on an industrial scale. Furthermore, they are not generally continuous and require relatively expensive equipment. This method also does not remove any other mineral detritus which may be present.
It is also known to eliminate the pyritic sulfur by transforming it to sulfuric acid but such techniques are environmentally unsound and dangerous. They also cannot be readily applied with lost cost apparatus which can be integrated into a conventional processing line for the coal.
Thus efforts to remove pyrites from coal in a practical way and on a larger scale have concentrated upon wet methods utilizing hydrocyclones and flotation methods or the like. These methods also allow recovery of different classes of coal for different purposes. Nevertheless even these methods have not been found to be practical in all cases especially when the coal, pyrite and mineral matter are interlaced and are intimately bound to one another.